He treks up mountains and rises at dawn for freezing swims in the ponds at Hampstead Heath. At 67, Nicholas Phillips is fit and up for a challenge. He'll need to be: next month Lord Phillips of Worth Matravers will succeed Lord Woolf as lord chief justice, the top judge in England and Wales and the main buffer between the judges and a government gearing up for fresh court battles.

The past weeks have seen politicians insinuating that the judges can be blamed if we are not as secure from terror attacks as we should be, and judges vowing that they will not be browbeaten. This week the home secretary, Charles Clarke, finally hoisted the white flag, telling the Prison Reform Trust: "It is always dangerous for a politician to tread in any way on the toes of the judiciary, as I have already discovered."

The truce is unlikely to last. Lord Phillips takes over at a time when trust between judges and politicians is at a new low. Politicians blame "judicial activism" for frustrating the will of parliament. Judges have been spooked by what they see as attempts to bring them into line over the deportation of terrorist suspects to countries with dubious human rights records and by constitutional reforms weakening the role of the lord chancellor, their traditional protector. The Constitutional Reform Act 2005, when it comes into force next year, will give the lord chief justice a much bigger role, require the lord chancellor to swear to respect the rule of law and put ministers under a duty to uphold the independence of the judiciary.

Lord Woolf, universally liked for his personal warmth, was a champion of judicial independence and the rule of law. Phillips, who steps up from the number two job, master of the rolls, is a different character. One QC said: "He's rather reserved, not exactly shy but very much a man to himself. He wouldn't be instinctively a cuddly man. He is very much an archetypal judge: upright, with total integrity, a nice person without wanting to put his arms around somebody and cuddle them." Will he be as effective in fighting the judges' corner? "I think Nick will handle politicians more astutely than Harry [Lord Woolf] did."

But with proposed legislation telling judges how to interpret the Human Rights Act - requiring them to balance the interest in state security against the individual's right not to be tortured - human rights lawyers question whether Phillips will be a match for the lord chancellor, Lord Falconer, an astute operator and Tony Blair's right hand man, and Clarke, under pressure to look tough.

Jeffrey Jowell, professor of public law at University College London and a former neighbour of Lord Phillips in Hampstead, points to his deft chairing of the contentious inquiry into the BSE crisis as evidence of his political skills and says his court judgments reveal a man not inclined to defer overmuch to ministers. "He pointed a finger where it ought to have been pointed. He didn't go over the top but suggested a number of procedures which could be improved. He has shown himself to be politic.

"When he started judging under the Human Rights Act he was one of those judges who understood the new constitutional position: that he didn't have to defer to the government simply because they were elected by the people, that the judges had a certain independent role. On the other hand, he wasn't unreasonable. He would allow the government a certain measure of discretion."

Like most senior judges, he comes from a public school/Oxbridge background, though he was evacuated to Canada during the second world war.

He built a commercial practice and was appointed a high court judge in 1987. In 1995 he presided over the Maxwell fraud trial, winning praise for his innovation, fairness, attention to detail and patience. He was promoted that year to the court of appeal and three years later elevated to the House of Lords. In 2000 he was made master of the rolls, heading the court of appeal's civil division.

He pushed through reforms spearheaded by Woolf to make civil justice cheaper and speedier. A moderniser, he pressed for more money for the courts, even going on BBC's Newsnight, a rare step for a senior judge, to air some of the problems in the civil justice system. He argued for wigs to be scrapped in civil cases, called the judges' multiple changes of garb ridiculous and expressed a preference for a French-style robe fastened with Velcro. He voiced concerns that fear of compensation claims made councils too risk averse: "Individuals should not be restrained from sporting activities that involve risk, like hang-gliding or swimming."

He once described the three-year BSE inquiry as "the most gruelling task I have ever had, by a long way". Two or three years from now, he may have reason to revise that view. "We're moving into times when he'll have to be as strong as Harry Woolf was," says Jowell.